1. Field of the Invention
The present invention relates generally to a method and means of radar signal processing for direction finding purposes, and, more particularly, to such a method and means especially advantageous for use with a radar target seeker.
2. Discussion of Related Art
A radar seeker operates generally by emitting radar beam pulses toward a target, measuring the time traveled by pulses reflected from the target and adjusting the radar beam for maximum response, which enables both the direction and distance of the target to be determined.
In the usual situation, there may be other objects adjacent the sought after target which will produce radar reflections and, in that way, induce confusion and error into the tracking system. Also, in a military context defensive measures are taken to intentionally interfere with the operation and accuracy of a radar seeker aboard, say, an aircraft. For example, large quantities of radar reflecting foil strips ("window") dropped in the vicinity of a flying target aircraft can effectively block out radar detection of the craft. Another frequently employed radar camouflaging technique for an aircraft consists of having one or more escort craft flying near the target aircraft which direct "jamming" radar beams of appropriate frequency toward the search radar source to confuse and induce spurious direction information into the search tracking system.
Direction-finding techniques based upon eigenstructure methods have been proposed and experimentally verified and have shown themselves to be superior to conventional direction-finding equipment for overcoming standard defensive measures. Application of eigenstructure techniques requires a radar system having an active antenna array, that is, a plurality of antennae arranged in a matrix for sending and receiving radar pulses over a relatively large area including a sought after target and which antennae are controllable as to phase and gain.
A more detailed discussion of a prior eigenstructure method can be found in the article, "Eigenstructure Methods for Direction Finding with Sensor Gain and Phase Uncertainties" by Anthony J Weiss and Benjamin Friedlander, Proceedings IEEE, ICASSP 198, New York, N.Y. This technique requires at least two sources (i.e., two reflected radar signals) for proper operation which excludes applicability to a very frequently encountered situation, namely, a single-source encounter. Moreover, in this and in all known prior eigenstructure techniques precise knowledge of signals received by the sensor array is required which, in turn, requires initial calibration of the entire seeker data collection system, a time consuming and difficult task. Still further, there is the necessity for maintaining array calibration in these known systems which is additionally difficult and time consuming.